High frequency measuring resistor



April 29, 1941. R. GOLICKE ETAL 2,239,781

HIGH FREQUENCY MEASURING RESISTOR Filed July 9, 1938 2 Sheets-Sheet 1 RA wk 52, Volts I I 1 an! 200 100 M I April 29, 1941.

R. GOLICKE EIAL HIGH FREQUENCY MEASURING RESISTOR 2 Sheets-Sheet 2 FiledJuly 9, 1958' Patented Apr. 29, 1941 UNITED STATES HIGH RESISTQE,

Roman Golicke,

, near Falkensee,

Application July 9, H38, Serial No. 218,4153 In July 15, 193$? Thepresent invention relates to high-frequency resistance means inparticular for measuring purposes.

As is well known the usual step resistors arranged in decimal connectionpresent the draw-- back when taking measurements at high Irequency thattheir self-capacity leads to false test readings and must be eliminatedby complicated computation and approximation methods. Owing to theirconstruction it is not possible to remove the self-capacity in suchresistors and the problem of designing a high-frequency measuringresistor for frequencies up to 20 megacycles having a very smallcapacity could not hitherto be solved. The value which the switchingcapacity of a highfrequency measuring resistor should have if the faultsas to phase and magnitude should not exceed a tolerance of i1% willhereinafter be explained by reference to an example. For the mostfavorable selection of the range of resistance in the case of a d'ecadicregulation of 50 to 5009, the switching inductivity amounts to 0.5 L2;-hem-y and the switching capacity Ci 6 fared or for f=20 megacycles andu7=1.25) 10 we obtain L24 nh., whereby 1 1111. (nanohenry) =lllhem-y areby a multiple order of magnitude greater than would be convenient, andit is also not possible to reduce these values to a considerable extenteven in the case of a compact construction and of special measures.

According to the invention it is proposed to employ as a measuringresistor, particularly for high-frequency measuring purposes, a materialwhose specific conductance may be continuously varied within wide limitswith the aid of :a suitable control magnitude. In this manner allswitching devices are avoided, thus eilecting a considerable saving inspace and wiring materials.

In modern electrical engineering numerous materials or elements areknown whose specific conductance may be easily varied extremely widelimits. In the present case it is panticuiarh Nil. 1375-363}advantageous to employ so-called thermonegative resistors, since thespecific conductance thereof increases considerably with increasingtemperature. Thennonegative resistors of a special type a are made ofpressed or burnt metal oxides, particularly of uranium dioxide. Thespecific conductanoe of such a resistor may therefore he varied by itstemperature which in turn controlled by an electrical control magnitude.

1m in principle. other heating conductors such as 20 the heating currentsupplied.

Figs. 2 to 4 represent circuit diagrams for simultaneously carrying outhigh and low frequency measurements. I

Fig, represents a measuring circuit in which the entire high frequencymeasuring resistor system is located in one branch or a low frequencyauxiliary measuring bridge, and

Fig, 6 represents a further improvement over the circuit Fl 5, in whichthe heating energy for the high frequency resistor is derived from theoutput of an amplifying system, whose input is connected to theauxiliary bridge.

Before indicating .a method for measuring high frequencies by the use ofa measuring resistor 5 according to the invention the properties, forinstance, of an uranium dioxide resistor will first he explained byreference to the accompanying drawings. In Fig. 1 are indicated by thecurrentvoltage characteristic 1] (dash curve) and the no resistancecharacteristic R (heavy line curve) of such a resistor in dependence onthe heating current 1h. The diagram shows that the resistance may bevaried between 20 and 15000 in steps so that, {or instance, a decadiccontrol may be easily 45 eiiected between 56 and 5009. However, from inthousandth steps.

The adjustment or such a resistor by reference to a calibration curvedetermined lay measuring the heating current such as is shown, forinstance, in Fig. l, is not suitalsle for accurate measure- 55 ments,since the temperature is also dependent upon the room temperature, etc.Consequently, a desired resistance value must be first checked or anadjusted resistance value must be checked afterwards with direct currentor low frequency or still better as is proposed according to theinvention the high-frequency and 1ow-frequency measurements are carriedout simultaneously. In this case the faults are eliminated which enterwhen the measurements are made at different times, during which theresistance may for instance have cooled. To this end, it is necessary toseparate the high-frequency circuit from the heating or the testingcircuit as will now be explained with reference to the arrangements ofFigs. 2 to 4.

According to Fig. 2 two approximately equal resistors 2R, each havingthe ohmic resistance 2R, are parallel connected for the high-frequencyso as to result in a resistance of the value R, whereas for the heatingcircuit Ih they are series-connected so as to result in a resistance ofthe value R. A condenser C is connected across the resistors 2B, andcompletes the high-frequency circuit so that both circuits havesubstantially only the ohmic resistance of the resistors 2R, in commonbut are otherwise separated. The heating current has a low frequency andmay be supplied from a utility network with alternating current ofcustomary frequency, or from a DC source. The

condenser C has a high impedance for such low frequency (or DC) currentsand thus does not interfere with the operation of the heating circuitnor form an operative part thereof. In this. case tolerances for thevalues of both resistors of are admissible, for the series-connectionthen still gives the fourfold value of the parallel connection exactlywithin i%%. In Fig. 3 is shown another embodiment which in principle issimilar to that shown in Fig. 2 except that here four individualresistors each having a resistance R are employed. The high-frequencycircuit is also here separated from not-illustrated part of the heatingcircuit Ih by a capacitor C. The two embodiments above-described havethe drawback that the two branches containing the resistors are notsymmetrical with respect to the highfrequency measuring circuit. Asymmetrical arrangement of the high-frequency resistors is shown in Fig.4 in which two resistors, each of the resistance value 2R, lie in twobranches which are connected to earth through two capacitors each of themagnitude 2C,

The simplest complete circuit connection suitable for a measuringresistor according to the invention is shown in Fig. 5. Here the entirehigh-frequency measuring resistor system designated by HFR, and which isconnected at the terminals HF to the high frequency system to beinvestigated (not shown here), lies in one of the branches of alow-frequency auxiliary measuring bridge and in order to establish thedesired resistance value the resistance standard N is adjusted to thisvalue and the heating current is regulated by a rheostat Rh until abridge balance is brought about. Conversely, in balancing ahigh-frequency bridge the heating is at first regulated and then theresistance standard is so adjusted that the auxiliary bridge isbalanced. The heating current derived from source B serves at the sametime as measuring current for the auxiliary bridge.

According to the invention the separate adjustment of the regulatingresistor Rh and of the standard resistor-N in Fig. 5 may be avoided andthe operation may be simplified particularly by deriving the heatingenergy from the output side of an amplifying arrangement whose inputside is connected to and controlled by the auxiliary bridge; 1. e., thevoltage existing across the neutral or zero branch of the auxiliarybridge circult is employed for controlling the increase or decrease ofheating. In this manner the measuring resistor depending upon theregulating slope or sensitivity of the arrangement adjusts itselfautomatically within 10.1% or within an even smaller fraction of thestandard resistance.

Such a connection is shown in Fig. 6. The high-frequency measuringresistor system HFR, to the terminals HF of which the high frequencysystem to be investigated is connected as in Fig. 5, lies as abovedescribed also in one of the branches of the auxiliary bridge circuit,while the input circuit of an amplifying arrangement, consisting of aplurality of amplifier tubes V'Rl, VH2, ADI and AD! and of thecorresponding transformers Ui, U2, U3 and the conventional capacitorsand resistors cooperating therewith, is connected to the zero branch l-2of the bridge. In this case it is preferable to feed the auxiliarybridge circuit with alternating current, particularly with alternatingcurrent from the utility network. To the output side of the lastamplifying stage ADI, AD2. which receives its anode energy from thealternating current source by way of transformer Tr, is connected thetransformer U3, the secondary winding of which supplies the heating andoperating current for the measuring bridge, this current being composedof a voltage component taken from the supply network and a voltagecomponent taken from the anodes of the ADI and ADZ tubes. Consequently,a modulation of these two voltages occurs which may be added orsubtracted at the zero branch of the auxiliary bridge depending upon theposition of the phase, thus effecting automatically upon a change of thestandard resistance N in either direction the raising or lowering of thehigh-frequency resistor temperature and therefore a readjustment of thehigh-frequency resistor to the value of the resistance standard. Thetubes ADI and ADI therefore act in this connection as a controlledregulating resistor, similar to Rh in Fig. 5, located between the energysupply source and the current consuming device (bridge). In the case ofan accurate bridge balance these tubes are therefore not loaded or onlyto a slight extent and onlyan average heating energy is supplied to theauxiliary bridge circuit or the high-frequency measuring resistance. If,however, the bridge circuit is detuned the output of the tubes isvaried, and it depends upon the phase displacement between the controlvoltage and the supply voltage whether in this case the resistance ofthe tubes increases or decreases. For instance, if the control voltageis in phase with the anode voltage (voltage of supply network) theresistance decreases and the heating of the measuring resistor increasesand vice versa.

Since the phase of the voltage in the bridge branch reverses dependingupon whether the measuring HFR resistance is greater or smaller than thestandard resistance N, theheating may be increased by suitably changingthe polarity of the transformer U3 if the measuring resistance is toogreat or the heating may be decreased if the measuring resistance is toosmall.

If, for instance, the amplification is chosen so high that already adetuning of :0.1% varies the heating from 0 to the maximum value and ifthis maximum value amounts to. a multiple of the fixed maximum value, itis possible to make the measuring resistance HFR. automatically equal tothe standard resistance N within 10.1

This may under certain circumstances take several seconds to attainowing to the heat sluggishness ofthe positive resistor, but on the otherhand this sluggishness avoids hunting of the regulating actions of theamplifying system. To reduce the great heat sluggishness of the knownresistor material it is preferable to use resistor shapes which have anincreased surface as compared with their volume, such as tubularresistors instead of rod resistors. plifying system, for instance, thevoltage applied to the grids of the end tubes AD! and A132 is a measurefor the degree of the bridge balance preveiling at that moment. Afterchanging the standard resistance it is thereforeadvisable to wait untilthe filament power of the amplifiers, after temporarily assuming a 'veryhigh or low The loading of the amvalue, has returned again to an averagevalue.

In order that this may easily be controlled 2. direct-current instrumentJa is inserted in the anode circuit, which instrument then indicates thefilament power, since the anode curent is proportional to the latter. Inthis way an agreement between the measuring and the standard resistancewithin at least 20.1% is ensured.- Owing to the above-mentioned heatsluggishness of the positive resistor its cooling downdepends upon itstime constant; The fine balancing of the bridge is nevertheless eifectedin a relatively short time, since the filament power is already cut 05when detuning the bridge within 0.1%.'

However, the heating up of the resistor may be accelerated at will bythe use of a greater filament power.

If in the case of high-frequency measurements a large number ofmeasurements must be carried out with a constant resistance'value it maybe under certain circumstances preferable to bring the resistor in athermostat controlled chamber to the temperature corresponding to thedesired resistance value and to maintain the chamber temperatureconstant by the thermostat. -,However, this arrangement requires agreater cost.

The high-frequency resistor is preferably so designed that thehigh-frequency part proper. see Figs. 5 and 6, is separated from thecontrol circuit or if desired it may be combined with the high-frequencymeasuring bridge to an instrument. This feature forms no part of thepresent invention and is therefore not illustrated. The high-frequencymeasuring instrument is then connected to the amplifying regulatingarrangement by a flexible conductor. It is also preferable to mount thematerial for the resistor so that its circuit connections and terminalshave the smallest possible capacity. Also this is a feature which per seis not involved in the present invention and pertains rathertoexpedients conventional in the mounting and connections of elementsforming part of a high frequency circuit. Its detail illustration hastherefore also been omitted.

While we have shown and described a particular mode of heating the highfrequency resistors, it is obvious that the heating per so maybeaccomplished by other equivalent means within the scope of the inventionso long as the heating is controlled according to the idea described andclaimed.

What is claimed is:

1. In combination with a high-frequency circuit, ohmic measuringresistance means connected in said circuit for high-frequencymeasurements and consisting of a material having a temperatum-responsive electric resistance varying with temperature over a widerange, a measuring bridge circuit including said resistance means in oneof its branches, circuit means forming part, of said high-frequencycircuit for separating said high-irequencycircuit from the otherbranches of said bridge circuit so that said two circuits havesubstantially only said ohmic resistance means in common, a standardresistance connected in one of said other of said bridge circuitbranches, and a current source included in said auxiliary bridge forheating said resistance means to a temperature necessary to produce aresistance value determined by said standard resistance when the bridgecircuit is balanced.

2., In combination with a high-frequency circuit in combination, avariable resistance device comprising a plurality of ohmic resistanceelements connected in parallel in said high-irequency measuring circuitand consisting of a material having a temperature-responsive specificresistance such that their resistance value varies continuously withtheir temperature over a wide range, and a heating circuit connectedwith said resistance elements so as to include said elements in seriesconnection and-containing controllable means for supplying saidresistance elements with variable heating current for varying theirtemperature to enable their adjustment to a desired resistance value,and circuit means forming part of said high-frequency circuit andcompleting said latter circuit independent of said controllable currentsupply means.

3. In combination with a high-frequency measuring circuit incombination, a measuring resistance arrangement comprising a pluralityof ohmic resistance elements connected in parallel in saidhigh-frequency measuring circuit and being composed of a material havinga temperature-responsive specific resistance such that their resistancevalue varies continuously with their temperature over a wide range, amea'suring'bridge circuit including said resistance elements in seriesin one of its branches a standard resistor arranged in another branch ofsaid bridge, current supply means included in said bridge circuit forheating said resistance elements to a temperature necessary to adjusttheir resistance to a value determined by said standard resistor whensaid bridge circuit is balanced, and circuit means forming part of saidhigh-frequency circuit and completing said circuit separately from saidstandard resistor and said current supply means. v 4. In combinationwith a high-frequency measuring circuit in combination, a measuringresistance arrangement consisting of a plurality of resistance elementsconnected in parallel in a high-frequency measuring circuit and beingcomposed of a material having a temperature-responsive resistance suchthat their resistance value varies continuously with temperature over awide range, a plurality of condensers forming part of saidhigh-frequency circuit, each of said condensers being connected to oneend of said resistance elements respectively and having a common groundconnection together with the other c0n densers symmetrical with respectto said elements, and a heating circuit connected to the ends of saidelements to which said condensers are connected and including all ofsaid elements in series for varying their temperature to enable theiradjustment to a desired resistance value during the high-frequencymeasurement.

5. In combination a measuring resistance Ior high-frequency measuringcircuits, consisting of a plurality of resistance elements connected inparallel in a high-frequency measuring circuit and being composed of amaterial having a tempasture-responsive resistance such that theirresistance value varies continuously with temperature over a wide range,a plurality of condensers forming part of said high-frequency circult,each of said condensers being connected to one end of said resistanceelements respectively and having a common grolmd connection togetherwith the other condensers symmetrical to said elements, a measuringbridge circuit including allot saldresistanceelementsinseriesinone orits branches, a standard resistor arranged in another branch of saidbridge circuit, and current supply means also included in said bridgecircuit for heating said resistance elements to a temperature necessaryto adjust their resistance to a value determined by said, standardresistance when said bridge circuit is balanced.

6. In combination a measuring resistance for high-frequency measuringcircuits, consisting of a plurality of resistance elements connected inparallel in a high-frequency measuring circuit andbeingcomposedofamaterialhavingatempasture-responsive resistance suchthat their resistance value varies continuously with temperature over awide range, a plurality of condenser-s forming part of saidhigh-frequency circuit, each of said condensers being connected to oneend of said resistance elements respectivelyandhavingacommongroundconnectiontogether with the other condenserssymmetrical to said elements, a measuring bridge circuit includingallofsaidresistanceelementsinseriesinone of its branches. a standardresistor in. another branch ofsaldbridgedrcuihcurrentsuppty means alsoincluded'in said bridge circuit for heating said resistance elements toa temperature n to adjust their to a value determined by said standardresistance when said bridge circuit is balanced, and means forcontrolling the healing current in accordance with the potential in theneutral bridge branch said bridge circuit so that the smallest meanpotential prevails in said neutral bridge branch.

7. In combination a measuring resistance Ior high-frequency m circuits,ot a plurality oi resistance elements connected in parallel in a hih-frequency circuit and being composed of a material having atemperature-responsive resistance such that their resistance valuevaries continuously with temperature over a wide range, a plurality ofcondensers forming part of said high-frequency circuit, each or saidcondensers being connected to one end of said resistance elementsrespectively and having a common ground. connection together withtheother condensers symmetrical to said elements, a bridge circuitincludingall. of said resistance elements in series in one or itsbranches, 2. standard resistor in another branch of said bridle circuit,AC supply means for providing heating energy for said resistanceelements and operating energy for said bridge circuit, an amplifiersystem having its input circuit connected with the neutral bridgebranch, of

said bridge circuit, a second amplifier system having its input circuitconnected to the output sideotsaidflrst amplifier-system andhavingiisown output side electrically coupled with said AC supply means and withthe supply terminals of said bridge to produce two different synchronoussupply potentials, one modulating the other, so that the heating energysupplied to said bridge terminals is controlled by a resultant voltagedepending upon the phase positions of the voltage across said neutralbridge branch and the voltage of said supply means. a

8. In combination a measuring resistance for high-frequency measuringcircuits, consisting of a plurality of resistance elements connected inparallel in a high-rrequency measuring circuit and being composed of amaterial having a temperature-responsive resistance such that theirresistance value varies continuously with temperature over a wide range,a plurality of condensers forming part of said high-frequency circuit,each of said condensers being connected to one end of said resistanceelements respectively and having a common ground connection to etherwith the other condensers symmetrical to said elements, a measuringbridge circuit including all of said resistance elements in series inone of its branches, at standard resistance in another branch of saidbridge circuit, AC supply means for providing heating energy for saidresistance elements and operating energy for said bridge circuit, anamplifier system having its input circuit connected with the neutralbridge branch of said bridge circuit, a second amplifier system havingits input circuit connected to the output side of said first amplifiersystem and having its own output side associated with said AC supplymeans, a transformer connected into said amplifier output side andhaving its secondary winding connected with the supply terminals of saidbridge circuit to produce two dil- Ierent synchronous supply potentials,one modulating the other, so that the heating energy supplied to saidbridge terminals is controlled by a resultant voltage depending upon thephase positions of the voltage across said neutral bridge a branch andthe voltage of said supply means,

and means for varying the polarity of said transformer so as to increasethe heating when the resistance of said elements is too large and todecrease the heating when said resistanceis too small, depending uponthe character of the resistance material employed.

9. In combination with a high-frequency circuit, an ohmic measuringresistance unit for high-frequency measurements consisting of a materialhaving a substantially purely ohmic resistance varying continuously withtemperature over a wide range, said resistance having a large surface ascompared with its volume so as to have av minimum thermal inertia, andadjustable circuit means electrically connected with said resistance soas to form a heating circuit for controlling the temperature of saidresistance unit to adjust it to a desired resistance value during thehigh-frequency measurement, said highi'requency circuit being completelyindependent of said adjustable circuit means so as to have practicallyonly said ohmic resistance unit in common with said heating circuit.

10- In combination with a high-frequency circuit to be adjusted, acontrolling circuit including means for supplying auxiliary current oflower frequency, ohmic resistance means arranged in said two circuits tobe simultaneously operative therein and forming the only common part ofboth, said resistance means comprising a variable resistor of a materialhaving a temperature-responsive resistance varying with its temperatureover a resistance range of several orders of magnitude, said controllingcircuit containing an adjustable standard resistor and having a circuitbranch voltage dependent upon the resistance values of said tworesistors. and circuit means operatively connected between said circuitbranch and said current supply means for controlling said auxiliarycurrent in dependence upon the voltage of said circuit branch so as toheat said variable resistor to adjust it to a desired resistance valuein accordance with the adjustment of said standard resistor.

11. In combination with a high-frequency circuit, variable resistancemeans consisting of material having a specific resistance varying independence upon temperature over a resistance range of several orders ofmagnitude, a controlling circuit associated with said resistance meansand having current supply means for heating said resistance means, saidcontrolling circuit containing an adjustable standard resistor andhaving a circuit branch dependent as to voltage upon the resistance ofboth said resistor and said resistance means, and circuit meansoperatively connected between said circuit branch and saidcurrent supplymeans for controlling the heating of said resistance means in dependenceupon the voltage of said circuit branch to adjust said resistance meansto a desired resistance value in accordance with the adjustment of saidstandard resistor.

12. In combination with a high-frequency circult, two thermonegativeresistors parallel-connected in said circuit, a heating circuitconnected with said resistors so as to contain them inseries-arrangement, control means in said heating circuit for varyingthe heating current sup- J plied to said resistors to adjust theirresistance to a desired value, said high-frequency circuit includingshunt means of high impedance to said comprising a resistor consistingof a material having a temperature-responsive electric resistance suchthat its specific resistance value variescontinuously with temperatureover a wide range, variable control means associated with said resistorfor controlling the temperature of said resistor to selectively adjustsame to a predetermined resistance value effective in said highirequencycircuit.

14. In combination, a high-frequency circuit, an ohmic resistance deviceconnected in said high-frequency circuit and consisting of materialhaving a temperature-responsive specifloresistance which variescontinuously over a wide range withcorrespondingly varying temperature,an auxiliary circuit-for a lower frequency connected with said resistorso that said resistor forms a common element of said two circuits, 9.variable control resistance arranged in said auxiliary circuit forvarying the low-frequency current flowing through said ohmic resistancedevice in order to adjust its resistance to a desired value, variableimpedance means also arranged in said auxiliary circuit for setting saiddesircdresistance value, and indicating means also connected in saidauxiliary circuit for comparing the setting of said impedance with theresistanceof said resistance means as adjusted by said controlresistance.

15. In combination with a high-frequency circuit ohmic resistance meansconnected in said circuit for high-frequency measurements and consistingof a material having a thermonegative resistance v rying continuously iperature over a wide range, a measuring circuit connected with saidhigh-frequency circuitso as to have said ohmic resistance means incommon therewith, current-supply means connected with said measuringcircuit for energizing it with current of lower frequency, control meansassociciated with said resistance means for varying the temperature ofsaid resistance means to it to a desired resistance value, and measuringmeans in said measuring circuit for indicating the resistance value ofsaid, resistance means.

ROMAN GOLICKE. ANDREAS JAUMANN.

